In numerous structures, and in particular in machine frames, it is necessary to damp the vibration generated in the structure by various periodic or aperiodic disturbances which may be internal or external to the structure, and which may be due to the presence of members moving in translation or in rotation, for example, or to hydrodynamic or aerodynamic effects.
It is common practice to remedy this by making use of passive dampers of hydraulic, pneumatic, or hydropneumatic type, or even by making use of internal or mechanical friction. However, the damping provided by such devices is not sufficiently fast or complete in countering the vibration that may appear on a structure.
Proposals have also been made to mount the structure to be stabilized directly on an active magnetic suspension, as in U.S. Pat. No. US-A-4 244 629 for example, which relates to stablizing a mass in order to damp oscillations that may occur in a horizontal plane. In this case, the mass to be stabilized is fixed to two elementds which constitute the rotors of radial magnetic bearings and which are loosely mounted inside two radial magnetic bearing stators which are fixed to the ground. Such mounting is not designed to damp vibration in a vertical direction, and it requires a pair of radial magnetic bearings to be used, each of which requires two pairs of electromagnets, together with the corresponding servo-control loops, and it also requires the bearing stators to be mounted on the ground or on the supporting platform and the bearing rotors to be mounted on arms connected to the mass to be stabilized. Such an implementation which is relatively complex to implement and which provides effective damping in one plane only is therefore suitable for special applications, only.
European patent specification No. EP-A-0 157 693 describes a method and apparatus for reducing vibration in rotary machines fitted with active magnetic suspension. This document requires at least one conventional active radial magnetic bearing to be incorporated inside the very structure to be stabilized, which structure can be constituted only by the frame of a rotary machine, with the stator of the radial magnetic bearing being fixed to the frame while the rotor of the radial magnetic bearing is fixed to the rotary shaft of the machine. Such vibration-reducing apparatus is also limited to special applications in which the vibrations to be damped are repetitive in nature. In addition, the incorporation of magnetic bearing components inside the machine to be stabilized can give rise to assembly operations which may be quite complex when the machine was not originally fitted with a mangetic suspension.
There also exist some applications where it is necessary, on the contrary, to generate vibrations in a structure, said vibrations being of predetermined frequency and amplitude, and being directed along various directions in three dimensional space. In such a case, a set of unidirectional vibrators may be applied to the structure, but in practice, the use of a plurality of distinct vibrators turns out to be complex and the resulting vibrations are difficult to monitor and control.
The present invention seeks to remedy the above drawbacks and to provide damping in a simple and effective manner by means of a universal and compact apparatus for damping the vibrations that may occur on a sturcture without it being necessary to change the internal organization of said structure, or on the contrary to excite a structure with vibrations along various directions by means of a single apparatus outside the structure.